Engines, particularly gasoline engines, use a distributor to distribute high voltage electricity to cylinders, or more specifically spark plugs associated with the cylinders, for ignition. The transmission of high voltage electricity from the distributor to the cylinders or the spark plugs is done via high voltage cables, which are usually called ignition cables or ignition wires. A conventional ignition cable is shown in FIG. 4, which usually comprises an outer insulation sheath 92 generally made of rubber of silicone material in which a stretching resistant central core 94, usually made of a bundle of fiber material to provide resistance to stretching. A conductive intermediate layer 96, which is a mixture of graphite and rubber, is interposed between the outer sheath 92 and the central core 94 to transmit the high voltage electricity for ignition.
A problem associated with conventional ignition cable of this kind is that it possess a very high impedance, for example 16 k.OMEGA..+-.20% per meter length as indicated in JIS code C3409. The cables that connect to different cylinders of an engine usually have different lengths which results in a great difference in the overall cable impedance for different cylinders. For example, assuming the first cylinder of an engine takes an ignition cable of one meter long and that of the fourth cylinder half meter long, the difference in overall impedance between these two cylinders may be as high as 8 k.OMEGA. per meter length, if taking the value of JIS code C3409. Such a great difference in impedance may lead to a non-uniform power output of the engine.
To solve such a problem, attempts have been made to decrease the impedance per unit length of the ignition cable. However, the decrease of the impedance leads to increase in electro-magnetic radiation which causes interference with the operations of computer-based control system and radio.
Further, the conventional ignition cable structure mentioned above also has an aging problem after one or two years service. As a consequence of aging, ignition may become poor and thus the ignition cables have to be replaced for maintaining proper operation and efficiency of the engine.
It is therefore desirable to provide an engine ignition cable which is capable to overcome both the impedance and electro-magnetic radiation problems of the prior art structure.